High-speed imaging devices (high-speed video cameras) for taking consecutive images of high-speed phenomena such as explosions, destructions, combustions, collisions and discharges for only a short period of time have been conventionally developed (for example, refer to Non-Patent Document 1 and other documents). Such high-speed imaging devices need to perform an ultrahigh-speed imaging operation that exceeds a level of approximately one million frames per second. Accordingly, they use solid-state image sensors capable of high-speed operations, which have special structures different from those of the imaging devices used in normal video cameras, digital cameras and similar devices.
As an example of this type of solid-state image sensor, a device called an in-situ storage image sensor (IS-CCD), which is disclosed in Patent Document 1 and other documents, has been generally known. An outline of this image sensor is as follows: A storage CCD, which also serves for the transfer of a specified number of recorded images (frames), is provided for each photodiode as a photo-receiver. During an imaging operation, pixel signals resulting from photoelectric conversion by the photodiode are sequentially transferred to the storage CCD. After the imaging operation is completed, the pixel signals corresponding to the specified number of record frames stored in the storage CCD are collectively read, and the images corresponding to the specified number of record frames are reproduced outside the image sensor. During the imaging operation, pixel signals exceeding the specified number of image frames are discarded beginning from the oldest ones. Thus, the latest set of pixel signals corresponding to the specified number of frames are held in the storage CCD. This means that, when the transfer of pixel signals to the storage CCD is suspended at the completion of the imaging operation, one can obtain the latest series of images spanning from the completion of the imaging operation back through a period of time corresponding to the specified number of record frames.
In the aforementioned high-speed imaging, it is important to perform the imaging in synchronization with the timing of the occurrence of a phenomenon under observation. This is achieved by a control process in which the imaging action is discontinued or initiated in response to an externally given trigger signal. To generate this trigger signal, the system normally includes another sensor, such as a contact sensor, position sensor, vibration sensor or pressure sensor, in addition to the imaging device. However, in some situations, it is often difficult to obtain appropriate trigger signals by this method, as in the case where the sensor cannot be easily placed close to the object, where the imaging action must capture a spontaneous change in the object under observation, or where the imaging object is a micro-sized object under a microscope.
When, as in the aforementioned cases, it is difficult to obtain an appropriate trigger signal from another sensor, it is desirable to generate a trigger signal by detecting a movement of or change in an object from images of that object. For a relatively low-speed imaging operation, a device for detecting a movement of or change in an object by real-time image processing of acquired images and for generating a trigger signal has been already developed. In this type of device, the reference clock for driving the solid-state image sensor is raised to a level several times to several tens of times the imaging rate required for the actual imaging (i.e. it is operated in an over-sampling mode), and a real-time motion detection of an object is performed on the images reproduced from the image signals read from the solid-state image sensor. When, a movement of or change in the object is detected, a trigger signal is generated, whereupon the true imaging action is initiated or discontinued and the control of some devices other than the imaging device is performed (e.g. an illuminating device for imaging is turned on or off).
However, in the case of detecting a movement of or change in an object in real-time by image processing of the signals read from the image sensor, it is normally necessary to preserve the images in an external frame memory and perform a large amount of computation, which tends to increase the delay time from an occurrence of an objective phenomenon to the generation of a trigger signal. Particularly, in the case of high-speed imaging, this delay time may prevent the objective phenomenon from being captured in the resultant images.
To address these problems, an imaging system disclosed in Patent Document 2 uses a light-splitting means, such as a beam splitter or half mirror, provided behind the imaging lens. The light-splitting means separates incident light into plural beams, which are respectively introduced into two imaging devices, i.e. the first and second imaging devices. The first imaging device is dedicated to the monitoring to detect a sudden change in the image. According to a trigger signal generated by this imaging device, the acquisition of image signals in the second imaging device is controlled. This type of conventional imaging system requires optical parts to split incident light coming from the imaging object into plural beams, and additionally needs more than one imaging device (image sensor). Thus, the system will be large and complex, making it difficult to reduce the production cost and decrease the size and weight of the system.
Patent Document 3 discloses a system in which a change detection element dedicated for detecting a change in the amount of incident light is provided within and/or around the pixel area of a solid-state image sensor, and the initiation and discontinuation of imaging is controlled on the basis of a detection signal of the change detection element. However, providing the change detection element within the image sensor area inevitably lowers the quality of the resultant images since the pixel signals required for the imaging cannot be obtained at the portion where the change detection element is present. In one configuration that has been proposed to avoid this problem, the change detection element is located apart from the image sensor area, and a prism, half mirror or similar device is provided to split incident light into separate beams, which are respectively cast onto the change detection element and image sensor area. However, as in the conventional technique described in Patent Document 2, using an optical system for light-splitting purposes significantly increases the production cost. Furthermore, detecting a change that occurs within a portion of the imaging range requires a considerable number of change detection elements, which increases the chip area of the solid-state image sensor and accordingly increases the production cost.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-345441
Patent Document 2: Japanese Unexamined Patent Application Publication No. H05-336420
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2007-166581
Non-Patent Document 1: Kondo et al., “Kousokudo Bideo Kamera HyperVision HPV-1 no Kaihatsu (Development of “HyperVision HPV-1” High-Speed Video Camera))”, Shimadzu Hyouron (Shimadzu Review), Shimadzu Hyouron Henshuu-bu, Sep. 30, 2005, Vol. 62, No. 1/2, pp. 79-86